Steam generation using recycled water feedstock is a conventional practice during, for example, operations directed to recovering heavy oil from tar sands or other geological formations. The steam is utilized for heating the target hydrocarbons, thereby reducing their viscosity and increasing the mobility of hydrocarbons within the geological formation or other matrix in which it is naturally distributed. Prior art systems have incorporated a number of steam generator configurations including, for example, once-through type steam generators (OTSGs). As generally utilized in the industry, however, OTSGs tend to operate with relatively high blowdown rates, often in the range of from about 20% to 30%, resulting in substantial thermal budget and chemical treatment inefficiencies. Also, OTSGs are most commonly configured so that the steam is generated from the feed water stream in a single-pass operation through boiler tubes that are heated by gas or oil burners. OTSGs are typically operated to produce steam at pressures of from about 1000 pounds per square inch gauge (psig) to about 1800 psig while utilizing feed water that can have about 2000 mg/L to about 8000 mg/L of total dissolved solids (TDS).
The use of OTSG for SAGD applications requires a series of vapor-liquid separators to produce the requisite steam quality. For both SAGD and non-SAGD applications, pre-treatment of the OTSG feed water has consisted of silica reduction in a hot or warm lime softener, filtration and hardness removal by Weak Acid Cation (WAC) ion exchange. In most cases, the OTSG blowdown is disposed by deep well injection. As the use of SAGD increased, the traditional produced water treatment and steam generation methods were re-evaluated to determine whether alternative methods may provide more technically and economically viable solutions. One such alternative, the use of vertical-tube Mechanical Vapor Compression (MVC) evaporation, has rapidly become the “baseline” approach against which other technologies are evaluated. In addition, the method allows the use of standard or “packaged” drum boilers in lieu of OTSG for steam generation, providing further technical and economic benefits. In order to suppress silica scaling in the evaporator, conventional practice, as reflected by the operating conditions recommended by the evaporator manufacturers, is to maintain the aqueous solution at a relatively high pH of about 13 or more using sodium hydroxide and/or other base(s), to maintain silica solubility.
The conventional practice of operating at high pH is not, however, sufficient to maintain clean deposit-free heat transfer surfaces, particularly in systems in which substantial calcium and/or magnesium are introduced into the evaporator with the makeup water. The source of evaporator makeup can be a combination of surface water and increasingly brackish water sources are being utilized. Even when operating in the recommended elevated pH range, such systems tend to remain susceptible to the formation of calcium carbonate and/or magnesium silicate deposits which impede heat transfer and are difficult to remove, typically requiring time consuming off-line chemical and mechanical cleaning to restore the heat transfer surface. Depending on the makeup water composition, hydrocarbon fouling may also be a concern and may further reduce the system's heat transfer efficiency. And finally, the large amount of sodium hydroxide (caustic) or other bases consumed in the process of maintaining the system in the high pH target range can constitute a substantial expense and complicate waste water treatment as well.
In addition to the conventional high pH operation, another method of reducing the likelihood of scaling within such systems has been proposed whereby the silica concentration of the feed water is reduced before entering the evaporator. One such method utilizes a sorption chemical added to an evaporator assembly arranged upstream of the main boiler/steam generator. According to the disclosure materials, the sorption chemical also removes a portion of the calcium and magnesium from the system, thereby permitting utilization of high hardness and/or saline makeup water while reducing the caustic demand relative to conventional operations.
FIG. 1 depicts a conventional SAGD water treatment system 100, utilizing an evaporator system 102 configured for producing steam that may, in turn, be utilized as a boiler feed stream for producing the high quality steam utilized in steam injection systems. The high quality steam is injected through one or more steam injection wells 104, typically in combination with other injectant compositions, for fluidizing the heavy oil formation(s) 106 such as the heavy oils found in tar sand formations. As the injected steam cools and condenses, an oil/water mixture 108 is produced and begins to migrate through the formation toward one or more oil/water gathering wells 110, through which the oil/water mixture is pumped to the surface. The recovered oil/water mixture is then sent through an oil/water separator 112 in the mixture is separated into an oil-rich fraction 114 and an oil-contaminated aqueous fraction 116. The aqueous portion is typically subjected to an additional de-oiling process 118 to produce a de-oiled water stream 120 that can become part of the steam generator feed stream after additional treatment.
The disclosed method is directed to the treatment of the de-oiled water stream 120 and any additional makeup water or feed streams 122, 122′ before the various feed streams are introduced into the steam generator and/or the monitoring and treatment of the water within the steam generator itself.
As noted above, many conventional SAGD operations utilize OTSGs for creating the steam necessary the oil recovery operations. These OTSGs may include some provision for some initial treatment of the feed water stream. The water treatment is typically configured to ensure that the TDS value of the feed water is below a target maximum value (typically within about 8,000 to about 12,000 parts per million (ppm) (frequently reported as CaCO3 equivalents)) and that the feed water meets various other specific water treatment parameters, e.g., pH, before the water can be fed into the OTSGs for generating high pressure steam.
Accordingly, in most prior art water treatment schemes, the de-oiled water is subjected to costly treatment(s) in a water treatment sub-system before it can be sent to the steam generators. The treatments performed within the sub-system may include, for example, warm lime softeners for removing hardness as well as the addition of other softening chemicals including, for example, lime, flocculating polymer(s), and/or soda ash. The softener operation is frequently followed by filtration for reducing any carry-over of precipitate(s) or other suspended solids and a “polishing” operation utilizing ion-exchange, e.g., a weak acid cation (WAC) ion-exchange system, for removing additional hardness and reducing the associated alkalinity. As will be appreciated by those skilled in the art, these softener and ion exchange systems require regeneration chemicals and generate additional waste streams. Additional discussion of such systems may be found, for example, in Heins' U.S. Pat. No. 7,967,955, the contents of which are hereby incorporated, by reference, in their entirety.
For SAGD processes, one hundred percent (100%) quality steam is generally preferred for well injection (i.e., no entrained liquid water present in the injected stream) that tends to make the use of OTSGs problematic. Specifically, in order to produce 100% quality steam from an OTSG, a vapor-liquid separator must be used for separating the liquid water from the steam prior to injection. The liquid fraction extracted from the separator is then typically flashed in a series of flash tanks to recover a series of lower pressure steam flows which may be utilized for other plant heating purposes. After the last flashing stage has been completed, the residual hot water blowdown stream must then be handled, typically by recycling and/or disposal.
In summary, the conventional and widely utilized methods for treating heavy oil field produced waters in order prepare them for use in the feed stream for high quality steam generator operations are not entirely satisfactory. In particular, the conventional physical-chemical treatment process schemes are usually quite extensive, are relatively difficult to maintain and require significant operator attention. Further, the conventional physical-chemical treatment processes necessitate the use of a number of chemical additives which may comprise a considerable operating expense, may require special attention for safe handling and produce substantial quantities of undesirable sludge(s) and other waste streams, the disposal of which is increasingly difficult and/or expensive as the result of increasingly stringent environmental and regulatory requirements.
It is clear that the development of a simpler, more cost effective approach to water treatment in connection with high quality steam generation, particularly with respect to evaporator operation, would be desirable, particularly in connection with SAGD operations for heavy oil production. The new water treatment method(s) disclosed herein, and various embodiments thereof, can be successfully applied to a range of industrial applications including, for example, heavy oil production operations, for improving the evaporator operation.
Other important objectives, features, and additional advantages of the various embodiments of the novel process disclosed herein will become apparent to the reader from the foregoing and from the appended claims and the ensuing detailed description, as the discussion below proceeds in conjunction with examination of the accompanying drawing.